Sanitize some WAL-logging buffer handling in GIN and GiST code

As transam's README documents, the general order of actions recommended
when WAL-logging a buffer is to unlock and unpin buffers after leaving a
critical section.  This pattern was not being followed by some code
paths of GIN and GiST, adjusted in this commit, where buffers were
either unlocked or unpinned inside a critical section.  Based on my
analysis of each code path updated here, there is no reason to not
follow the recommended unlocking/unpin pattern done outside of a
critical section.

These inconsistencies are rather old, coming mainly from ecaa4708e5
and ff301d6e69.  The guidelines in the README predate these commits,
being introduced in 6d61cdec07.

Author: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/CALdSSPgBPnpNNzxv0Y+_GNFzW6PmzRZYh+_hpf06Y1N2zLhZaQ@mail.gmail.com

src/backend/access/gin/gindatapage.c

@@ -1854,10 +1854,10 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
 		PageSetLSN(page, recptr);
 	}
 
-	UnlockReleaseBuffer(buffer);
-
 	END_CRIT_SECTION();
 
+	UnlockReleaseBuffer(buffer);
+
 	/* During index build, count the newly-added data page */
 	if (buildStats)
 		buildStats->nDataPages++;

src/backend/access/gin/ginfast.c

@@ -134,10 +134,10 @@ writeListPage(Relation index, Buffer buffer,
 	/* get free space before releasing buffer */
 	freesize = PageGetExactFreeSpace(page);
 
-	UnlockReleaseBuffer(buffer);
-
 	END_CRIT_SECTION();
 
+	UnlockReleaseBuffer(buffer);
+
 	return freesize;
 }
 
@@ -459,10 +459,10 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 	if (metadata->nPendingPages * GIN_PAGE_FREESIZE > cleanupSize * (Size) 1024)
 		needCleanup = true;
 
-	UnlockReleaseBuffer(metabuffer);
-
 	END_CRIT_SECTION();
 
+	UnlockReleaseBuffer(metabuffer);
+
 	/*
 	 * Since it could contend with concurrent cleanup process we cleanup
 	 * pending list not forcibly.
@@ -659,11 +659,11 @@ shiftList(Relation index, Buffer metabuffer, BlockNumber newHead,
 			}
 		}
 
+		END_CRIT_SECTION();
+
 		for (i = 0; i < data.ndeleted; i++)
 			UnlockReleaseBuffer(buffers[i]);
 
-		END_CRIT_SECTION();
-
 		for (i = 0; fill_fsm && i < data.ndeleted; i++)
 			RecordFreeIndexPage(index, freespace[i]);
 

src/backend/access/gin/ginutil.c

@@ -663,9 +663,9 @@ ginUpdateStats(Relation index, const GinStatsData *stats, bool is_build)
 		PageSetLSN(metapage, recptr);
 	}
 
-	UnlockReleaseBuffer(metabuffer);
-
 	END_CRIT_SECTION();
+
+	UnlockReleaseBuffer(metabuffer);
 }
 
 /*

src/backend/access/gin/ginvacuum.c

@@ -224,12 +224,12 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 		PageSetLSN(BufferGetPage(lBuffer), recptr);
 	}
 
+	END_CRIT_SECTION();
+
 	ReleaseBuffer(pBuffer);
 	ReleaseBuffer(lBuffer);
 	ReleaseBuffer(dBuffer);
 
-	END_CRIT_SECTION();
-
 	gvs->result->pages_newly_deleted++;
 	gvs->result->pages_deleted++;
 }
@@ -654,8 +654,8 @@ ginbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 			PageRestoreTempPage(resPage, page);
 			MarkBufferDirty(buffer);
 			xlogVacuumPage(gvs.index, buffer);
-			UnlockReleaseBuffer(buffer);
 			END_CRIT_SECTION();
+			UnlockReleaseBuffer(buffer);
 		}
 		else
 		{

src/backend/access/transam/xloginsert.c

@@ -1355,11 +1355,12 @@ log_newpage_range(Relation rel, ForkNumber forknum,
 		recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI);
 
 		for (i = 0; i < nbufs; i++)
-		{
 			PageSetLSN(BufferGetPage(bufpack[i]), recptr);
-			UnlockReleaseBuffer(bufpack[i]);
-		}
+
 		END_CRIT_SECTION();
+
+		for (i = 0; i < nbufs; i++)
+			UnlockReleaseBuffer(bufpack[i]);
 	}
 }